With the development of optical fiber sensing technology, application of optical fiber sensing range is more wide, the optical fiber is sensor element or transmission path. In some constructions, some fiber, from the point of functional design, just for light transmission, but form the point of principle of the structure, it has an ability to sense the physical quantities outside world. For example, FIG. 1 is an optical fiber sensing device for voice transmission (Reference: Su Ke, Jia Bo and so on, all-fiber audio transmission system and the security, optoelectronics • Laser [J], 12, 2005), wherein, 1,2,3,4,5 are ports of 3*3 coupler, 6,7,8 are ports of 2*2 coupler, 9 is a transmission optical fiber, 10 is coated with a highly reflective end surface the rate of the Al film microphone; 3*2*3 coupler and the coupler 2, optical fiber delay line, the transmission fiber 9 and a highly reflective Al film interference structure intact. In this configuration, there are two-way optical transmission paths interfere with each other, respectively:1→4→6→8→9→10→9→8→7→5→2(3)  I1→5→7→8→9→10→9→8→6→4→2(3)  II
Wherein, “→” indicates the direction of beam propagation. The structure belongs to a single feedback structure: two light transmitting direction opposite to interfere, from the output port 8 of the optical transmission along the optical fiber 9 to the end, the feedback means after the action (in this case, having a reflective film microphone 10), back to the optical fiber 9, the optical fiber 9 along the reverse transfer. Apparatus shown in FIG. 1, the sensing element for the Al film—when sensing sound, it will produce a corresponding vibration; function 9 transmission fiber for transmitting only the Al film reflected light from the function, the only transmission, but not for induction. However, constituting interference from the structure, the transmission fiber every point, have the ability to external vibration sensor 9, that is, when the vibration caused by external optical fiber transmission occurs at a point 9 on the corresponding physical changes, interference signal will produce a corresponding change. 9 transmission fiber sensing signal is superimposed on the sensor signal with the Al film, Al film interference signals induced, therefore, signal transmission fiber 9 sensing harmful signals, from the perspective of system functions, it is undesirable for.
Thus, in practice, in order to reduce sensing functions have, but only as a function of the optical fiber transmission pick up disturbing signals along its layout, I hope this part of the fiber is protected, placed in a relatively quiet environment. This solution obviously increases the complexity of the optical fiber laid. Moreover, in some applications, only use already laid optical fiber (cable) as a transmission path, when this part of the environment in which the cable is difficult to meet the desired state of the environment, the sensing performance of the system greatly reduced, or even not work properly.